In recent years, along with a general trend of reducing the size and the thickness of electronic equipments, it has been strongly demanded to decrease the size and reduce the thickness of the electronic parts or power source devices used for them.
On the other hand, LSI, for example, in CPU have been improved in the operation speed and increased in the degree of integration and a high current is sometimes supplied to a power supply circuit for LSI. Accordingly, it is necessary for inductors such as choke coils used in the power source circuit described above to lower heat generation by decreasing the resistance of coil conductors and suffer from less lowering of the inductance value due to DC super impose (i.e., satisfactory DC superimposing characteristics).
Further, since the working frequency for them has become higher, it is also necessary that the loss in a high frequency region is low.
Further, since it has been strongly demanded for reducing the cost of parts, it is necessary that a device of a constitution with a simple shape can be assembled in a simple step. That is, it has been demanded to provide inductors or power source modules usable at high current and high frequency and reduced in the size and the thickness at a reduced cost. Among various kinds of parts used for the power source circuits, an inductor has a greatest thickness. In view of the above, it has been strongly demanded for reducing the thickness of a magnetic device such as an inductor in order to decrease the thickness of a power source module.
However, as the size of the magnetic device is decreased, the magnetic channel cross sectional area is generally decreased to lower the inductance value. An example of improving the characteristics of such a small sized magnetic device, i.e., a method of increasing the inductance value, is disclosed in Japanese Patent Laid-Open application No. S61-136213.
In this case, windings are applied to a flanged drum-shaped core material using ferrite or the like and then the inside of flanges is filled with a mixture of a magnetic powder and a resin to form a closed magnetic channel structure. In this structure, a bobbin usually used for windings is no more necessary to increase the magnetic channel cross sectional area by so much and attain a closed magnetic channel structure. As a result, the inductance value increases and the characteristic of the magnetic device is improved. However, the structure is intended for the size reduction of the magnetic device but not intended for the reduction of the thickness. Further, since the magnetic channel length in a mixture of the magnetic powder and the resin is long, it can not be said that sufficient characteristic can be obtained and it still leaves a subject.
Further, the ferrite material as a magnetic material used most generally has a relatively high permeability and a saturation magnetic flux density is lower compared with that of metal magnetic materials. As a result, when it is used as it is, the inductance lowers greatly by magnetic saturation tending to worsen the DC current superimpose characteristic. Then, for improving the DC superimposing characteristic, a gap is usually formed to a portion of the magnetic channel of a ferrite core thereby lowering the apparent permeability in use. However, since the saturated magnetic flux density is low, it is difficult to cope with high current.
Next, in a case of using, for example, an Fe—Si—Al alloy, Fe—Ni alloy, or the like having a saturation magnetic flux density higher than that of ferrite as the core material, since the metal materials described above have low electric resistance eddy current loss increases and, they can not be used as they are.
On the other hand, a dust core prepared by molding a metal magnetic powder has a remarkably high saturation magnetic flux density compared with a soft magnetic ferrite. Accordingly, this is excellent in the DC superimpose characteristics and advantageous in the size reduction. Further, since it is not necessary to provide a gap, it has a feature free from the problem of beats.
However, the dust core involves a problem that the core loss is large.
The core loss includes hysteresis loss and eddy current loss.
The eddy current loss increases in proportion with the square of frequency and square of the size in which the eddy current flows. Further, the dust core material is usually molded at a molding pressure of several tons/cm2 or more. Accordingly, since the permeability is deteriorated along with increase in the distortion as the magnetic body, the hysteresis loss increases. In order to overcome the problem, occurrence of the eddy current is suppressed by covering the surface of a metal magnetic powder with an electrically insulating resin or the like. Further, for solving the problem of the hysteresis loss, strains are released by applying a heat treatment after molding. Examples of the countermeasures described above are disclosed, for example, in Japanese Patent Laid-Open Application Nos. H6-342714, H8-37107 and H9-125108.
However, for coping with further higher current, higher frequency and lower profile for power sources, the existent constitution involves a problem that it can not sufficiently insure the lower resistance, higher inductance value or high frequency characteristic of conductors.
The present invention intends to solve the foregoing subject and provides a magnetic device capable of obtaining a sufficient inductance value and an excellent in high frequency characteristic even when it is with smaller size and lower profile.